1. Field of the Invention
The present invention relates to an image rotating optical system capable of rotating the position of an image with respect to the optical axis, adapted for use for example in a microfilm reader or a microfilm reader-printer.
2. Description of the Prior Art
An optical system capable of arbitrarily rotating the position of an image about the optical axis of the imaging system is desired in various applications, for example a microfilm reader and a microfilm reader-printer. The image on a microfilm is usually recorded in a normal position but often is recorded in a vertically inverted position, and such an inverted image has to be corrected to the normal position by suitable means.
In order to rotate the image about the optical axis of an imaging system, there is already known an optical element for reflecting the light beam an odd number of times, and the rotation of a projected image can be achieved by rotating such optical element. FIG. 1 shows an image rotating optical system utilizing a dove prism as an image-rotating optical element of single reflection type, wherein are illustrated a projection lens 1, a screen 2, a pupil 3 of said projection lens facing said screen and passing a circular light beam as shown in FIG. 2, a dove prism 4 constituting the image-rotating optical element and provided with such a dimension as to accept the entire light beam passing through the pupil 3 without causing any eclipse in said beam. The light beam from said projection lens 1 is projected onto the screen 2 through the dove prism 4, wherein the image on the screen can be rotated by rotating said prism about the optical axis of the projection lens. Such image rotating element of odd-number-times of reflection type tends, however, to require long faces along the axial direction of the imaging system, thus leading to a larger dimension of such element and to an increased production cost. A smaller image rotating element, particularly reduced in the axial length L, is desirable in consideration of the geometrical design of the optical system and also of the production cost, but it has been found that such reduction in size leads to an uneven illuminance in the projected image.